I read the following and alot of people claim it helped a great deal on their performance, but I won't get to test it out until later this evening.

In your DOOM 3 directory, find "DoomConfig.cfg" (x:\Program Files\Doom 3\base). Open DoomConfig.cfg in Notepad.

Find the line:

seta image_cacheMegs "32"

I changed 32 to 96.
Quite a few people have raised it to 128 and even 256. Just curious if it has any effect for anyone here.

Thanks.